In recent years, desires have been voiced for venetian blinds with differently coloured slats for realizing different colour patterns and colour combinations in each individual venetian blind. Thus, it may be desirable, in one and the same venetian blind, to have a plurality of identical and/or different width fields with slats of different colours. For producing such a venetian blind in a per se known machine, it is necessary manually to change the slat belts, which entails a considerably longer manufacturing time and, thereby, a drastic increase in production costs. Hence, there is a need in the art for an apparatus for the automatic alternation or changing of slat belts in a venetian blind producing machine.